You have to remove him from your team and add him where you want. If say he has 89 rating in lf and thats where you have him and you just move him to 2b he will still have that 89 rating when maybe it should be 93. After you remove him and put him in 2b he will be a 93. I also believe if he is out of position he will have fielding penalties
the only way to improve him is to feed him